A bead mill method is known as a technique for mixing and/or disaggregating a fine ceramic powder (see, for example, patent document 1). Beads for use in bead mills are required to have high wear resistance in order to minimize contamination caused by the bead material as a result of bead wear (see, for example, patent documents 2 and 3).
A technique for producing a spherical powder using a thermal plasma has been disclosed as one method for enhancing the wear resistance of beads (see, for example, patent documents 4 and 5). In such techniques for producing a spherical powder using a thermal plasma, a radio-frequency plasma is in main use because the plasma is generated in a wide region (see, for example, patent documents 5, 6, and 7). In the radio-frequency plasma method, particles which have passed through a high-temperature region melt in the thermal plasma and become highly spherical powder particles. However, this method has had a problem that particles distributed in the region which surrounds the thermal plasma and through which a sheath gas is flowing remain unmelted and come to coexist as particles having low wear resistance. Consequently, investigations are being made, for example, on a method in which a raw-material powder is introduced into a direct-current (DC) arc plasma and blown off while being melted and the powder is thereby led to the high-temperature portion of a radio-frequency plasma (see, for example, patent document 4) and on a method in which a radio-frequency plasma is arranged in two stages (see, for example, patent document 5). However, these methods not only involve a complicated process but also have had a drawback that when the raw-material powder has pores therein, the particles burst upon melting or voids are formed through melting, which constitutes an obstacle that makes it difficult to sufficiently round the particles.